php-fpm: Some POST data missing when uploading a file

I'm using PHP 7.2 fpm with nginx on an Ubuntu 18.04 machine and I'm having a problem with form data getting lost. The form submits data roughly in this structure:

[
'video' => [
    'translations' => [
        'de' => [
            'title' => 'string',
            'subtitles' => 'upload field'
        ]
    ],
    'posterFrame' => 'upload field'
    'duration' => 'string'
]]

When I leave the upload fields empty, the entire form data is available correctly in $_POST. However, when I try to upload a file (10 kB in size) in the posterFrame field, it shows up as expected in $_FILES, but $_POST then only contains the keys that were listed before the upload field, i.e. in the example, translations is available in $_POST, but duration is missing (I also checked php://input, which is empty).

All the relevant php.ini settings (max_post_size, allow_file_uploads etc.) look fine, and the application code itself also seems ok, at least it works on a different 18.04 machine (with Apache/mod_php), so I'm at a loss as to what might be the problem here. The nginx vhost config is straight from the Symfony documentation, and php-fpm config is what shipped with Ubuntu. There's nothing in any of the error logs I checked (php-errors, syslog, nginx/error.log etc). What could have gone wrong here?

EDIT: There is one thing I just noticed: When I post the form, I get these lines in nginx/access.log:

[my client ip] - - [16/May/2018:14:56:52 +0000] "POST /backend/videos/edit/1162214/ HTTP/1.1" 500 11373 "http://[myserver]/backend/videos/edit/1162214/" "Mozilla/5.0"
[my client ip] - - [16/May/2018:14:56:52 +0000] "\xFF\xD8\xFF\xE0\x00\x10JFIF\x00\x01\x01\x01\x00H\x00H\x00\x00\xFF\xE1I%Exif\x00\x00II*\x00\x08\x00\x00\x00\x09\x00\x0F\x01\x02\x00\x06\x00\x00\x00z\x00\x00\x00\x10\x01\x02\x00\x0E\x00\x00\x00\x80\x00\x00\x00\x12\x01\x03\x00\x01\x00\x00\x00\x01\x00\x00\x00\x1A\x01\x05\x00\x01\x00\x00\x00\xA0\x00\x00\x00\x1B\x01\x05\x00\x01\x00\x00\x00\xA8\x00\x00\x00(\x01\x03\x00\x01\x00\x00\x00\x02\x00s32\x01\x02\x00\x14\x00\x00\x00\xB0\x00\x00\x00\x13\x02\x03\x00\x01\x00\x00\x00\x02\x003si\x87\x04\x00\x01\x00\x00\x00\xC4\x00\x00\x00X$\x00\x00Canon\x00Canon EOS 20D\x0033\x7F\xFF\xF5\xBB3333\xB7\xF7\xBF;\xB3;33H\x00\x00\x00\x01\x00\x00\x00H\x00\x00\x00\x01\x00\x00\x002018:05:06 18:33:31\x00\x1C\x00\x9A\x82\x05\x00\x01\x00\x00\x00\x1A\x02\x00\x00\x9D\x82\x05\x00\x01\x00\x00\x00\x22\x02\x00\x00\x22\x88\x03\x00\x01\x00\x00\x00\x02\x0073'\x88\x03\x00\x01\x00\x00\x00@\x06\xCC\xCC\x00\x90\x07\x00\x04\x00\x00\x000221\x03\x90\x02\x00\x14\x00\x00\x00*\x02\x00\x00\x04\x90\x02\x00\x14\x00\x00\x00>\x02\x00\x00\x01\x91\x07\x00\x04\x00\x00\x00\x01\x02\x03\x00\x01\x92" 400 182 "-" "-"

So at first glance I would say that this is the binary data of the image that somehow ends up in the request URL? How can that happen?

Solution

ok, after converting the setup to Apache/mod_php I noticed that the GD extension was not installed on this machine (got a Call to undefined method imagecreatefromjepg in the log). I installed GD, and now it works properly. No idea why php-fpm swallowed the error and produced such strange behavior in nginx though